This invention relates to a device for an X-ray CT apparatus, which eliminates a scattering beam in the slice direction of an X-ray.
In an X-ray CT apparatus, X-ray beam 12 from X-ray source 10 is formed through collimators 14 into a fan-shaped pattern, as shown in FIG. 1. The fan-shaped X-ray beam 12 has a proper circumferential length in fan-out direction 2 and a somewhat small width in slice direction 1. On the optical path of X-ray beam 12, a patient, not shown, lies on a bed and an X-ray passed through the patient enters into, for example, arcuate X-ray gas detector 16. X-ray gas detector 16 is curved in fan-out direction 2 with X-ray source 10 as a center. A high pressure xenon gas is sealed into X-ray detector 16 and plate-like electrodes, not shown, are arranged at a proper interval along fan-out direction 2 such that they are located in a substantially parallel fashion. Upon the entry of the X-ray into an area between the adjacent electrodes, a xenon gas is ionized to yield Xe ions and electrons. The Xe ions and electrons are detected by the corresponding electrodes as an ion current so that an amount of X-ray entering into the area between the respective electrodes is converted to an electric signal. In the X-ray CT apparatus, X-ray source 10 and X-ray detector 16 are rotated around a rotation axis which passes through the patient and is parallel to slice direction 1. Signals are produced in accordance with the amounts of X-ray during the rotation of X-ray source 10 and X-ray detector 16 and processed through computation to obtain a patient's slice image.
In such an X-ray CT apparatus, the X-ray from X-ray source 10 penetrates the patient and enters directly into detector 16. In addition to the X-ray beam, a scattering beam is also produced. Due to the scattering beam an error is introduced into the X-ray detection signal, impeding the formation of an exact image.
In fan-out direction 2, since the parallel electrodes of X-ray detector 16 function as a grid, the incidence of the scattering beam is suppressed to such an extent that it can practically be disregarded. In slice direction 1, however, the scattering beam is incident to gas detector 16 without being eliminated. In the prior art, no countermeasure has been taken so as to eliminate such a scattering beam in the slice direction. This is because, in the slice direction, an X-ray incident to X-ray detector 16 is one which has almost totally been transmitted through the patient, and less of the scattering beam is incident thereto. Recently, there is a tendency for the slice width of the X-ray to be decreased in order to improve spatial resolution in an X-ray CT image. However, a problem arises due to an increase in the penumbra area of the X-ray and in the amount of incident scattering beam.
FIGS. 2 and 3 are views each showing a relation between the X-ray beam in the slice direction and the area to which the X-ray beam is incident. In the prior art, since, as shown in FIG. 2, diaphragms 14 are located with a broader spacing therebetween and since X-ray beam 12 illuminated from the focus of X-ray source 10 has a broader width as a slice width S (the width taken in the slice direction of X-ray 12 at the position of the patient), penumbra area 24 is narrowed so that less dosage of X-ray is incident to X-ray detector 16. As shown in FIG. 3, however, if the slice width S of X-ray 12 is narrowed by an upper collimator, penumbra area 24 is increased and, for this reason, a greater amount of scattering beam 26 coming from the patient is incident to X-ray detector 16, thus degrading the resultant image.
In this way, if the slice width of the X-ray is narrowed so as to enhance the spatial resolution, an image degradation problem arises due to the scattering beam, thus prominently reducing the spatial resolution enhancement effect.